1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method, in which a primary transfer of once transferring a toner image formed on a photoreceptor to an intermediate transfer member is carried out and thereafter a secondary transfer of transferring a toner image formed on the intermediate transfer member to a recording medium is carried out.
2. Description of the Related Art
Generally, in an image forming apparatus using an electrophotographic method, images are formed through a charging step, an exposure step, a developing step, a transfer step, a fixing step or the like. At the charging step, a surface of a photoreceptor is homogeneously charged in a dark place. At the exposure step, an original image is projected on the charged photoreceptor and thereby, electric charges of a part of the photoreceptor which received light is removed, so that an electrostatic latent image is formed on the surface of the photoreceptor. At the developing step, a toner image is formed by depositing toner as a developer on the electrostatic latent image formed on the photoreceptor. At the transfer step, the toner image formed on the photoreceptor is transferred on a recording medium such as paper and a sheet. At the fixing step the toner image transferred on the recording medium is fixed by heating means, pressurizing means and the like. The image forming apparatus forms a desired image on the recording medium using the electrophotographic method.
It is known that there are mainly two types of color images forming apparatuses which form images with use of toners of a plurality of colors, for instance, toners of four colors such as black, cyan, magenta, and yellow, according to a difference of the transfer step in which toner images of a plurality of colors formed on a plurality of the photoreceptors are transferred to the recording medium such as paper.
One is an image forming apparatus in which toner images of a plurality of colors formed on a plurality of photoreceptors are sequentially transferred directly to the recording medium so that the toner images of the plurality of colors are laminated on the recording medium The color image forming apparatus is an apparatus in which every photoreceptor has a structure to transfer a toner image formed on the photoreceptor to the recording medium. In order to transfer the toner image to the recording medium, it is not necessary to add a new component and therefore, the apparatus does not considerably grow in size. However, even for printing in black-and-white, for instance, even for printing only in black, the recording medium must pass on all the photoreceptors and therefore its printing speed is the same as that in a case of a color printing. Thus, it is not possible to realize high-speed printing. Moreover, since the recording medium must pass on all the photoreceptors, the colors are easily mixed. Further, since the toner images of a plurality of colors formed on the photoreceptors are transferred directly to the recording medium at every photoreceptor, unless conveying speed of the recording medium is exactly controlled, misregistration occurs among the respective color images and it is difficult to adjust registration.
The other is an image forming apparatus in which toner images of a plurality of colors formed on a plurality of photoreceptors are sequentially transferred once on an intermediate transfer member to implement a primary transfer in which the toner images of a plurality of colors are laminated on the intermediate transfer member and thereafter, implemented is a secondary transfer in which the toner images of the plurality of colors are collectively transferred on the recording medium. In such an image forming apparatus, the toner images are laminated on the intermediate transfer member by sequentially transferring the toner images of the plurality of colors to the intermediate transfer member and thereafter, the toner images are collectively transferred to the recording medium. This makes it possible to realize high-speed printing and prevent colors from being easily mixed. Furthermore, without exactly controlling conveying speed of the recording medium, misregistration can be prevented from occurring among the respective color images and it is easy to adjust registration. Therefore, focus of attention has been directed to development of such a color image forming apparatus with use of an intermediate transfer member.
However, there has been a problem related to secondary transfer characteristics of a secondary transfer portion to tend to become unstable according to changes in surface resistance of an intermediate transfer belt caused by environmental factor and the like. An exemplary related art to solve the above-mentioned problem is described in Japanese Unexamined Patent Publication JP-A 5-333723 (1993). The intermediate transfer apparatus of JP-A 5-333723 (1993) is an intermediate transfer apparatus used in an image forming apparatus, in which a backup roller (a secondary transfer counter roller) is used as a conductive roller and further the backup roller is grounded.
Furthermore, there is known as an art similar to the art of JP-A 5-333723 an image forming apparatus having a backup roller grounded via a resister.
Furthermore there is known as another related art an image forming apparatus in which a transfer electric field is changed according to types of the recording medium during the secondary transfer.
However, the secondary transfer characteristics in a secondary transfer portion is adversely affected not only by a cause of fluctuation in surface resistance of the intermediate transfer belt as an intermediate transfer member but also by a cause of unevenness in layer thickness of the toner images laminated on the intermediate transfer member. The layer thickness of the toner images laminated on the intermediate transfer member is not even since, for instance, only unicolor toner images are transferred on some parts while toner images of three colors are transferred on other parts. Therefore, it is difficult to set a transfer condition which is a condition for implementing the secondary transfer. Specifically, optimum values of a transfer electric field which is applied to the recording medium in the secondary transfer differ between a case of a thin and a thick toner image layer on the intermediate transfer member, and it is difficult to set an optimum transfer electric field. That is to say, when the transfer electric field is determined based on a thin layer part of the toner image (a thin layer portion), a thick layer part of the toner image (a thick layer portion) is transferred with less transfer efficiency, and hue of the color images transferred on the recording medium becomes incompatible with desired hue. Besides, when the transfer electric field is determined based on the thick layer portion, the transfer electric field is too strong for the thin layer portion, and therefore, the recording medium is excessively charged with its excess electric field. Therefore, after the toner images formed on the intermediate transfer member are transferred to the recording medium, the recording medium is electrostatically attracted to the intermediate transfer member, with the result of defective peeling that the recording medium is not peeled off the intermediate transfer member. When the defective peeling occurs, the recording medium is conveyed under the condition that the recording medium is attracted to the intermediate transfer member and therefore, the recording medium deviates from a conveying path, which leads a conveying trouble of the recording medium due to the defective peeling, so-called a conveying jam. The conveying jam prominently occurs when the recording medium is thin paper.
In the image forming apparatus using the intermediate transfer member, the thickness of the toner image laminated on the intermediate transfer member is not even and therefore, it is difficult to optimize the transfer electric field which is applied to the recording medium when the toner image on the intermediate transfer member is transferred to recording medium. Therefore, it is desired that, according to the thickness of a toner image on the intermediate transfer member, without changing the transfer electric filed, adequate transfer efficiency with which the toner image on the intermediate transfer member is transferred to the recording medium is fulfilled and further, sufficient peeling performance is provided to peel the recording medium off the intermediate transfer member after the toner image on the intermediate transfer member is transferred to the recording medium.
In order to fulfill the adequate transfer efficiency and provide the sufficient peeling performance, it is necessary that transfer of the toner image between the recording medium and the intermediate transfer member can be carried out, and furthermore a condition for peeling the recording medium off the intermediate transfer member is necessary between the recording medium and the intermediate transfer member.
On the other hand, in the intermediate transfer apparatus of Japanese Unexamined Patent Publication JP-A 5-333723 (1993) or image forming apparatuses of other related arts, there lacks countermeasures according to types of recording medium and environmental changes (such as temperature and humidity) with respect to peeling performance and therefore, the peeling performance cannot sometimes be fulfilled.
Specifically, in a case where the recording medium is a regular paper or a heavy paper the transfer performance can be maintained by grounding the backup roller and then, no peeling trouble occurs since the recording medium is stiff, in other words, it has high stiffness. On the other hand, in a case where the recording medium is thin paper, even when the transfer performance can be maintained in a way that the backup roller is grounded, since the recording medium is not stiff, electrostatic attraction caused by a difference in potential makes the recording medium deviate from the conveying path, and it causes the peeling trouble that the recording medium is conveyed under the condition that the recording medium is attracted to the intermediate transfer member.